The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for detecting an end-point of a metal layer during a chemical mechanical polishing operation.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time.
One way to determine the polishing endpoint is to remove the substrate from the polishing surface and examine it. For example, the substrate may be transferred to a metrology station where the thickness of a substrate layer is measured, e.g., with a profilometer or a resistivity measurement. If the desired specifications are not met, the substrate is reloaded into the CMP apparatus for further processing. This is a time consuming procedure that reduces the throughput of the CMP apparatus. Alternatively, the examination might reveal that an excessive amount of material has been removed, rendering the substrate unusable.
Several methods have been developed for in-situ polishing endpoint detection. Most of these methods involve monitoring a parameter associated with the substrate surface, and indicating an endpoint when the parameter abruptly changes. For example, where an insulative or dielectric layer is being polished to expose an underlying metal layer, the coefficient of friction and the reflectivity of the substrate will change abruptly when the metal layer is exposed.
Where the monitored parameter changes abruptly at the polishing endpoint, such endpoint detection methods are acceptable. However, as the substrate is being polished, the polishing pad condition and the slurry composition at the pad-substrate interface may change. Such changes may mask the exposure of an underlying layer, or they may imitate an endpoint condition. Additionally, such endpoint detection methods will not work if only planarization is being performed, if the underlying layer is to be over-polished, or if the underlying layer and the overlying layer have similar physical properties.
In one aspect, the invention is directed to a method of polishing a substrate. A first layer of a substrate is chemical mechanical polished with a first polishing fluid. The substrate has a second layer disposed under the first layer, and the first and second layers have differing reflectivity. The substrate is optically monitored during polishing with the first polishing slurry to generate plurality of intensity traces. Each intensity trace includes intensity measurements from a different radial range on the substrate. Once any of the intensity traces indicates an initial clearance of the first layer, the substrate is chemical mechanical polished with a second polishing fluid having different polishing properties than the first polishing fluid. Optical monitoring of the substrate continues during polishing with the second polishing slurry, and polishing is halted after all the intensity traces indicate that the second layer has been completely exposed.
Implementations of the invention may include one or more of the following features. Optical monitoring may include directing a light beam through a window in a polishing surface and causing the light beam to move in a path across the substrate, monitoring a reflectance signal produced by the light beam reflecting off the substrate, and extracting a plurality of intensity measurements from the reflectance signal. Generating the plurality of intensity traces may include sorting each intensity measurement into one of the radial ranges according to a position of the light beam during the intensity measurement and determining the intensity trace from the intensity measurements associated with the radial range. The first slurry may be a high-selectivity slurry and the second slurry may be a low-selectivity slurry. The first layer may be more reflective than the second layer. The first layer may be a metal layer, such as copper. The second layer may be an oxide layer, such as silicon dioxide, or a barrier layer, such as tantalum or tantalum nitride.
In another aspect, the invention is directed to a method of polishing a substrate in which a surface of a substrate is brought into contact with a polishing surface that has a window. The substrate has a first layer disposed over a second layer, and the first and second layers have differing reflectivity. A first slurry is supplied to the substrate for a first polishing step, and relative motion is caused between the substrate and the polishing surface. A light beam is directed through the window, and the motion of the polishing surface relative to the substrate causes the light beam to move in a path across the substrate. A reflectance signal produced by the light beam reflecting off the substrate is monitored, a plurality of intensity measurements are extracted from the reflectance signal, and a plurality of intensity traces are generated with each intensity trace including intensity measurements from a different radial range on the substrate. A second slurry is supplied to the substrate for a polishing second polishing step when any of the intensity traces indicates an initial clearance of the first layer. The second slurry has different polishing properties than the first slurry. Polishing is halted after all the intensity traces indicate that the second layer has been completely exposed.
Implementations of the invention may include one or more of the following features. The first slurry may be a high-selectivity slurry and the second slurry may be a low-selectivity slurry. The second layer may be disposed over a third layer in the substrate.
In another aspect, the invention is directed to a method of polishing a substrate having a metal layer disposed over an oxide layer. In the method, a surface of a substrate is brought into contact with a polishing surface that has a window. A high-selectivity slurry is supplied to the polishing surface. Relative motion is caused between the substrate and the polishing surface. A light beam is directed through the window, and the motion of the polishing surface relative to the substrate causes the light beam to move in a path across the substrate. A reflectance signal produced by the light beam reflecting off the substrate is monitored, and a plurality of intensity measurements are extracted from the reflectance signal. A radial position is determined for each intensity measurement, and the plurality of intensity measurements are divided into a plurality of radial ranges according to the radial positions. A plurality of intensity traces are generated, with each intensity trace including intensity measurements from one of the plurality of radial ranges. A low-selectivity slurry is supplied to the polishing surface when any of the intensity traces indicates an initial clearance of the metal layer, and polishing is halted when all the intensity traces indicate that the oxide layer has been completely exposed.
Implementations of the invention may include one or more of the following features. A sudden drop in a reflectance trace may indicates initial clearance of the metal layer in the radial range associated with the reflectance trace. A flattening out of a reflectance trace may indicate exposure of the oxide layer in the radial range associated with the reflectance trace. The substrate may includes a barrier layer, e.g., tantalum or tantalum nitride, disposed between the metal layer, e.g., copper, and the oxide layer, e.g., silicon oxide.
Advantages of the invention include one or more of the following. A copper-coated substrate may be polished with reduced dishing and erosion. Both the polishing endpoint, and the point at which the polishing apparatus should switch from a high-selectivity slurry to a low-selectivity slurry may be accurately determined. Detailed data is available on the progress of the metal polishing operation at different locations of the wafer.
Other features and advantages of the invention will become apparent from the following description, including the drawings and claims.